This application is based on Japanese Patent Application No. 2000-149965 filed May 22, 2000, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a control apparatus for controlling an engaging force of a clutch disposed in series with a transmission in an automotive vehicle.
2. Discussion of Related Art
There is known a clutch disposed in series with a transmission in a power transmitting path between an engine and a drive wheel of an automotive vehicle. An example of this clutch is a clutch provided for directly connecting an input shaft and an output shaft of a torque converter or a fluid coupling.
In an automotive vehicle of the type indicated above, the clutch is controlled so as to gradually increase the engaging force of the clutch, in the process of an engaging action of the clutch from its fully released position to its fully engaged position, and the speed ratio of a continuously variable transmission of belt-and-pulley type is normally controlled in a feedback fashion while the clutch is held in its fully engaged position.
If the speed ratio of the transmission in the automotive vehicle described above is controlled during a feedback control of the clutch so as to gradually increase its engaging force in the process of its engaging action, however, the vehicle suffers from a drawback that the engaging force of the clutch and the speed ratio of the transmission cannot be adequately controlled due to an interference between the two controls. For instance, a change of the output speed of the clutch as a result of the control of the speed ratio of the transmission tends to cause reduced stability in the control of the engaging force of the clutch so as to control a difference between the speeds of the input and output shafts of the clutch. On the other hand, a change of the speed of the output shaft of the clutch as a result of the control of the engaging force tends to cause reduced stability in the control of the speed ratio of the transmission so as to permit the actual speed ratio to coincide with a desired or target value.
It is therefore an object of the present invention to provide a control apparatus for a clutch in an automotive vehicle, which control apparatus permits stable control of the engaging force of the clutch during the control of the speed ratio of a transmission of the vehicle.
The above object may be achieved according to the principle of this invention, which provides a control apparatus for feedback-controlling an engaging action of a clutch disposed between an engine and a transmission of an automotive vehicle, the control apparatus comprising: engaging force control means operable upon an engaging action of the clutch, for determining a control amount and feedback-controlling the engaging action on the basis of the determined control amount such that the clutch is placed in a desired state of engagement; and control amount limiting means operable when the determined control amount has changed to cause an engaging force of the clutch to be reduced, for limiting the determined control amount.
While the clutch is gradually engaged under the feedback control by the engaging force control means, the control amount must be changed by a relatively large amount to compensate for an external disturbance that causes the engaging force to be reduced. Where the external disturbance causes the engaging force to be increased, on the other hand, the amount of change of the control amount required to compensate for this external disturbance is comparatively small. In view of this fact, the control amount limiting means is arranged to limit the control amount determined by the engaging force control means, when the determined control amount has changed to cause the engaging force of the clutch to be reduced. This arrangement is effective to reduce an influence of a control of a speed ratio of the transmission on the feedback control of the engaging action of the clutch by the engaging force control means, so that the engaging force of the clutch can be controlled with a high degree of stability even while the speed ratio of the transmission is controlled.
According to one preferred form of the present invention, the transmission is a continuously variable transmission a speed ratio of which is continuously variable, and control apparatus further comprises shift control means operable while the engaging action of the clutch is feedback-controlled by the engaging force control means, for controlling the speed ratio of the continuously variable transmission such that an actual value of the speed ratio coincides with a target value. In this form of the control apparatus, the feedback control of the engaging action of the clutch by the engaging force control means and the control of the speed ratio of the continuously variable transmission by the shift control means are effected simultaneously.
According to another preferred form of the invention, the engaging force control means controls the engaging action of the clutch on the basis of an input shaft speed and an output shaft speed of the clutch. For instance, the engaging force control means controls the engaging action of the clutch such that a rate of change of a difference between the input and output shaft speeds of the clutch coincides with a predetermined target value. This arrangement permits adequate control of the engaging action or state of the clutch.
According to a further preferred form of the invention, the control amount limiting means permits the engaging force control means to control the engaging action of the clutch on the basis of the control amount as determined by the engaging force control means, when the control amount determined by the engaging force control means has changed to cause the engaging force of the clutch to be increased. This arrangement permits the engaging force of the clutch to be increased with a comparatively high degree of stability even while the speed ratio of the transmission is controlled at the same time.
According to a still further preferred form of this invention, the transmission is an automatic transmission having a plurality of operating positions having respective different speed ratios, and the control apparatus further comprises shift control means for automatically shifting the automatic transmission to a selected one of the operating positions. In this form of the invention, the feedback control of the engaging action of the clutch by the engaging force control means and the control of the shifting action of the automatic transmission by the shift control means to a selected one of the operating positions can be effected simultaneously.
According to a yet further preferred form of this invention, the automotive vehicle includes a torque converter incorporating a lock-up clutch as the clutch the engaging action of which is feedback-controlled by the engaging force control means, and the torque converter has an engaging fluid chamber and a releasing fluid chamber on opposite sides of the lock-up clutch. In this form of the invention, the lock-up clutch is operable between a fully engaged position and a fully released position, by flows of a working fluid into and from the engaging and releasing fluid chambers, and engaging force control means is arranged to determine as the control amount a desired difference between pressures of the working fluid in the engaging and releasing fluid chambers.